1. Field of the Invention
The present invention relates to apparatus for switchable attenuation of a differential input signal from a microphone, and a microphone pre-amplifier comprising the same.
2. Description of the Related Art
It is known to provide switchable attenuation in the signal path between a microphone and a microphone pre-amplifier, possibly forming part of a mixing console, to reduce the level of the input signal by typically 10 to 20 decibels. This is often desirable for very loud sources such as percussion to avoid clipping of the input signal in the pre-amplifier, or to enable the pre-amplifier to be operated in a certain range for creative reasons such as introducing particular kinds of distortion particular to that range of the amplifier.
Such attenuators, which are often referred to as microphone pads, typically only switch in a fixed series resistance. Thus, the level of the input signal immediately drops by a fixed amount, which is equivalent to modulation of the audio signal by a step function. This introduces severe transient distortion, which manifests as a thud or a pop, which can be commercially unacceptable. The attenuator is often switched by means of a relay, and thus whilst it is being switched in the amplifier may be automatically muted to avoid the thud or pop associated therewith.
It is therefore an object of the present invention to provide an improved switchable attenuator for a microphone which does not require the entire input to be muted.